Positron Injection into a Magnetic Dipole Field for the Study of an Electron-Positron Plasma

• Applicants: Privatdozent Dr. Uwe Hergenhahn, since 2/2019; Professor Dr. Christoph Hugenschmidt
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 285825712

Within this project we plan to produce a low-energy, high-brightness positron beam and to use it for the first creation of an electron-positron plasma. This plasma is expected to have unique properties compared to conventional plasmas, for example to be essentially turbulence-free. The uniqueness stems from the exact mass symmetry, in contrast to electron-ion plasmas. A novel magnetic field configuration will be used to confine the plasma, namely a magnetic dipole trap. Particle losses by annihilation are predicted to be sufficiently low to allow long confinement times, and still high enough that the annihilations can be used as a plasma diagnostic. Yet, up to now, no such plasma has been produced on Earth. We will exploit two state-of-the-art technologies to overcome the two main bottlenecks in the study of controlled pair plasmas: Insufficient number of positrons, and insufficient confinement. The NEPOMUC beamline at the FRM II research reactor in Garching, developed by the first applicant, is a unique device for the production of high-intensity positron beams using a nuclear capture reaction. NEPOMUC is currently the most intense source of slow positrons in the world. Excellent confinement of both neutral and non-neutral plasmas by the field of a levitated, superconducting current loop has in recent years been demonstrated by the second applicant.In this project, we plan to take the last missing step on the path to production of a pair plasma, namely the injection of the positron beam into the confinement region, and plan to carry out first studies of a plasma formed after successful injection. As a prerequisite, we will develop the extraction of positrons from the NEPOMUC source at a low DC bias in order to generate a low-energy positron beam suitable for injection. For injection, a strategy using deflection plates to induce ExB drifts on closed orbits will be combined with a 'rotating wall'-like AC field to stabilize the ensuing positron orbits. An alternative approach for positron injection will be carried out by using a tungsten single crystal for positron re-moderation immediately after the ExB filter. This method would allow us to easily separate the primary positron beam and the brightness enhanced remoderated positron beam in order to explore the potential for more efficient injection into the dipole field.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Dr. Haruhiko Saitoh, until 2/2019